mixsyn-error: Output argument "r12" (and maybe others) not assigned during call to "hinf_st"

Hey, I would like to solve the inverted pendulum control problem with the mixsyn-function but unfortunately I can't make it work.

The error says: Output argument "r12" (and maybe others) not assigned during call to "hinf_st"

Here is my code:

 M = .5; m = 0.2; b = 0.1; I = 0.006; g = 9.8; l = 0.3;
 p = I*(M+m)+M*m*l^2; %denominator for the A and B matrices

 A = [0      1              0           0;
     0 -(I+m*l^2)*b/p  (m^2*g*l^2)/p   0;
     0      0              0           1;
     0 -(m*l*b)/p       m*g*l*(M+m)/p  0];
 B = [     0;
     (I+m*l^2)/p;
          0;
        m*l/p];
 C = [1 0 0 0;
     0 0 1 0];
 D = [0;
     0];

 states = {'x' 'x_dot' 'phi' 'phi_dot'};
 inputs = {'u'};
 outputs = {'x'; 'phi'};

 sys_ss = ss(A,B,C,D,'statename',states,'inputname',inputs,'outputname',outputs);
 G = tf(minreal(sys_ss));

 M = 2; wB1 = 6; wB2 = 12; A = 1e-3;
 s = tf('s');
 wp1 = (s/M+wB1)/(s+wB1*A);
 wp2 = (s/M+wB2)/(s+wB2*A);
 wP = [wp1 0; 0 wp2];
 wU = 1;
 [K,CL,GAM,INFO] = mixsyn(G,wP,wU,[]);

any ideas?